The present invention relates to a laser exposure unit, and more particularly to a laser exposure unit wherein laser beams of at least two different wavelengths or more scan a recording medium to expose a color image for formation thereof.
As a laser exposure unit to expose a color image for formation thereof, there have been known a unit wherein gas lasers (an argon laser, a helium neon laser and others) for three wavelengths corresponding to B (blue), G (green) and R (red) and an audio-optical modulation element (hereinafter referred to as AOM) are used in combination and a unit having the structure wherein semiconductor lasers for three wavelengths are subjected to direct modulation (internal modulation).
However, an argon laser which is frequently used especially as B light source among gas lasers has had problems that a unit is big in size and is high in cost.
Further, in modulation by means of a conventional AOM, a laser beam emitted from a gas laser has been caused to enter AOM directly. In this case, an exposure speed is restricted by a modulation speed of AOM, which has made it impossible for the exposure speed to be enhanced sufficiently.
On the other hand, if light-sensitive materials sensitive to red wavelength or longer wavelength are used for three layers for C (cyan), M (magenta) and Y (yellow), it is possible to use a relatively inexpensive semiconductor laser for all light sources. However, the light-sensitive material having the aforesaid characteristics is hard to be designed, manufactured and preserved, a price of such light-sensitive material itself is high, which has been a problem. In addition, three wavelengths are close together due to the use of red wavelength and longer wavelength, causing a possibility of occurrence of an interference of adjoining wavelengths, which has been a problem.
The invention has been achieved in view of the problems mentioned above, and its first object is to provide a laser exposure unit which is compact in size and low in cost and which is further capable of exposing a color image stably at high speed.
Next, problems which are related to plural laser beams and are to be solved by the invention will be described as follows.
In an image recording apparatus having therein a light source section where plural laser diodes with the same wavelength are lined up, a beam lining up optical system which lines up laser beams emitted from the plural laser diodes on a recording material in the sub-scanning direction, and a moving means which moves the laser beams emitted from the plural laser diodes and the recording material relatively in the main scanning direction, adjustment of intensity of a laser beam has been conducted by controlling an electric current which flows through a laser diode.
However, in the image recording apparatus stated above, both of adjustment to make all laser beams to be in the same intensity and adjustment of exposure intensity for all laser beams emitted from plural laser diodes need to be conducted by controlling an electric current which flows through a laser diode. Though the adjustment to make all laser beams to be in the same intensity does not need to be conducted frequently, shifting of exposure intensity for all laser beans emitted from the plural laser diodes needs to be conducted surely when changing tone reproduction for outputted images and when changing a recording material, which requires frequent adjustment.
However, an electric current flowing through a laser diode and an amount of emitted light are not in a proportional relation, and the relation between an electric current and an amount of emitted light varies depending on each laser diode and the change of the relation is different depending on the passage of time. Therefore, a measuring means which measures beam intensity of each laser beam emitted from each of plural laser diodes has been required to be provided, and it has been necessary to adjust frequently an electric current that flows though each laser diode of plural laser diodes depending on beam intensity of each laser beam measured by the measuring means, when changing tone reproduction of outputted images and changing recording materials, which has been time-consuming.
It has been common that beam intensity of a laser beam from each laser diode is measured by an optical system, and thereby an electric current which flows through each laser diode of plural laser diodes is adjusted.
However, the foregoing has been effective only for adjustment for variation in beam intensity caused by variation of intensity of luminescence of laser diode with the passage of time, and it has been impossible to adjust variation of beam intensity caused by a change in an optical axis.
In the conventional unit, it has been necessary to cause a recording position of a laser beam from each laser diode to agree with the main scanning direction so that the recording position on a recording material may not be deviated. Therefore, accurate adjustment has been necessary for each unit before its installation or after usage thereof for a certain period of time.
The second object of the invention is to solve the problems related to plural beans and thereby to attain the following items. Shifting of exposure intensity for all laser beams emitted from plural laser diodes in the case of changing tone reproduction for outputted images required to be conducted frequently and changing recording materials is made to be carried out in a short period of time in a simple and stable way.
Variation of beam intensity of each laser beam including variation of beam intensity caused not only by variation of luminescence intensity of a laser beam with the passage of time but also by a change in an optical axis is made to be corrected satisfactorily.